The present invention relates to rubber compositions suitable for tire treads, and more particularly to rubber compositions suitable for use in treads of radial tires for passenger cars and treads of racing car tires.
In order to reduce running fuel cost of automobiles, decrease in rolling resistance of tires and lightening in weight of automobile parts including tire have hitherto been demanded as well as improvement in efficiency of engine and auxiliaries. It is the most effective for reducing the rolling resistance of tires to reduce the energy loss of rubbers used in respective components of a tire, particularly the energy loss of a rubber of tread which accounts for a large volume.
When natural rubber (NR) or butadiene rubber (BR) is used in order to reduce the energy loss of tread rubber, the rolling resistance is reduced, but the wet grip is simultaneously reduced. For such a reason, in recent years, solution-polymerized styrene-butadiene rubbers wherein the styrene content and the vinyl bond content are regulated within specific ranges, have been proposed and used, thereby achieving the reduction of the rolling resistance while maintaining or improving the wet grip characteristics of tires.
Also, in recent years, lightening of automobiles advances with development of improved engines, thus automobiles of low fuel cost having high performances have been developed. With the development of such automobiles, tires having a low rolling resistance, a high grip characteristic and a high steering stability have been increasingly demanded. For meeting such a demand, solution-polymerized styrene-butadiene rubbers having a higher than .delta. peak temperature (Tg) have been used with carbon black having a low energy loss in order to obtain high grip characteristics. In case of tires for racing cars which require to stably maintain a particularly high grip characteristic up to the latter stage of racing, a large quantity of a process oil such as aromatic oil is incorporated in addition to the use of the solution-polymerized SBR having a high Tg and the carbon black.
However, combination use of a solution-polymerized SBR having a high Tg with a carbon black having a low energy loss raises a problem that bagging (namely the phenomenon that a rubber winding on a roll rises to the surface of the roll, so cut and rubber sending operations become impossible), is easy to occur during processing steps, particularly on a warming mill such as load mill, surge mill or feed mill when warming a tread rubber prior to extruding it. The occurrence of bagging may make tire manufacturing impossible since it is difficult to feed a rubber to an extruder for tread production.
Also, in case of rubber compositions for racing car tires, since they contain a large amount of a process oil, the torque in a Banbury mixer does not rise when milling the compositions, thus bad dispersion of carbon black or other ingredients is easy to occur. This problem has been solved by remilling, but lowering of productivity is unavoidable.
It is an object of the present invention to provide a rubber composition suitable for use in treads of automobile tires and having a low rolling resistance, a high grip characteristic and a good processability.
A further object of the present invention is to provide a rubber composition suitable for use in treads of tires for automobiles, particularly radial tires for passenger cars, which has a low rolling resistance and a high grip characteristic, and further has a good processability such that it does not cause bagging during processing.
A still further object of the present invention is to provide a rubber composition suitable for use in treads of tires for racing cars, which has a particularly high grip characteristic and, moreover, which has an excellent milling workability.
Another object of the present invention is to provide a tire for automobiles having a low rolling resistance, a high grip characteristic, a high steering stability and an excellent processability.
These and other objects of the present invention will become apparent from the description hereinafter.